Rotary cylindrical mould members for moulding food products are well known in the art and commercially applied by Marel in the RevoPortioner since the late 1990's. Such a rotary cylindrical mould member is adapted to be used in a system for moulding food products from a mass of one or more food starting materials, the cylindrical mould member having a longitudinal axis and a peripheral cylindrical outer surface. The rotary cylindrical mould member comprises at least one porous mould body having a curved outer surface, forming at least part of the cylindrical outer surface of the mould member, and an opposite inner surface, which mould body is made at least partially from a homogeneous porous material, with a porous structure of intercommunicating pores opening out at the inner surface of the mould body.
The porous mould body comprises a curved outer surface which is sealed airtight; and one or more recessed mould cavities formed in the curved outer surface of the mould body, defining the shape of the products which are to be produced, the mould cavities being open in the outer surface of the mould body to allow for the filling of the mould cavity with a mass of one or more food starting materials. An open-pored cavity surface area defines the mould cavity in the known embodiments.
The rotary cylindrical mould member further comprises one or more air channels extending from an air inlet, adapted to provide communication from the air inlet via the air channel and via the porous structure of the porous mould body between the air channel and the open-pored cavity surface area into the mould cavities to assist in the removal of product from a mould cavity.
As indicated in WO2004/002229, the provision of mould cavities in a porous body by means of milling causes plastic deformation of the porous sintered stainless steel in a layer bounding the mould cavity, which generally causes the openings of the pores which open out at the surface to be completely closed up. In a subsequent manufacturing step at least part of the deformed cavity surface layer is treated by removing the deformed layer in its entirety, such that the pore openings are at the surface and thereby create an open-pored cavity surface area. In WO2004/002229, spark erosion is mentioned as a technique to cause the porous structure of the deformed cavity surface layer to be opened up again. Spark erosion, also referred to as electrical discharge machining (EDM), die sinking, wire burning or wire erosion, is a manufacturing process whereby material is removed from the workpiece by a series of rapidly recurring current discharges (sparks) between two electrodes, separated by a dielectric liquid and subject to an electric voltage. A known alternative technique to treat at least part of the closed pores at the surface is polishing, e.g. mechanically or chemically. A specific technique is electropolishing, also known as electrochemical polishing or electrolytic polishing, which is an electrochemical process that removes material from a metallic workpiece. It is used to polish, passivate, and deburr metal parts.
In the known techniques to treat at least part of the plastically deformed cavity surface layer, in which the openings of the pores which open out at the surface are substantially completely closed up, the plastically deformed cavity surface layer is essentially removed so as to expose the porous structure of the porous inner volume. By removing the skin layer formed by plastic deformation, the inner volume having a porous structure is exposed.
The regained porous structure and open-pored cavity surface layer bounding the cavity allows the removal of the food product from the one or more mould cavities by feeding compressed air from the air inlet via the air channel and via the porous structure of the mould body into the mould cavities.
Practical use of embodiments of the described rotary mould members, in particular when making food products out of ground red meat, experience undesired build-up of fat in the mould cavities, such that intermediate cleaning, e.g. by water jets, is required, for example every half hour, in particular frequently intermediate a shift of 16 hours. It is commonly desired to have a production time a shift of 16 hours prior to cleaning a mould drum overnight.